docs: def arcball_map_to_sphere(point, center, radius):
      """Return unit sphere coordinates from window coordinates."""
      v = numpy.array(((point[0] - center[0]) / radius,
                       (center[1] - point[1]) / radius,
                       0.0), dtype=numpy.float64)
      n = v[0]*v[0] + v[1]*v[1]
      if n > 1.0:
          v /= math.sqrt(n) # position outside of sphere
      else:
          v[2] = math.sqrt(1.0 - n)
      return v